The present invention relates to cobalt-base articles having high resistance to wear and corrosion in semi-solid metal environments. More specifically, the invention relates to fully dense powder metallurgy articles, made from a novel Co--Cr--W--C type alloy being particularly suited for long term use in high wear, high temperature machinery employing a variant process of semi-solid metal molding (SSM).
The metallurgical process referred to herein is one where metals and metal matrix composites are heated and stirred in the solid plus liquid phase region and then injected into a mold or die at lower temperatures. This process has proven to result in parts having improved material characteristics, previously uncastable and unobtainable shapes, and reduced post formation processing steps. Two versions of the above process, also known as Thixomolding.RTM. (Thixomat, Inc., Ann Arbor, Mich.), are generally disclosed in U.S. Pat. Nos. 4,694,881 and 4,694,882, which are herein incorporated by reference. The process generally involves the shearing of a semi-solid metal so as to inhibit the growth of dendritic solids and to produce nondendritic solids within a slurry having improved molding characteristics which result in part from its thixotropic properties (a semi-solid nondendritic material which exhibits a viscosity which is proportional to the applied shear rate and lower than that of the same alloy when in a dendritic state).
A machine adapted to employ the above type of processes and to which the present invention has particular applicability is schematically shown in FIG. 1. The construction of the molding machine 10 is, in some respects, similar to that of a plastic injection molding machine. In the illustrated machine 10, feed stock is fed via a hopper 12 into a heated, reciprocating screw injection system 14 which maintains the feedstock under a protective atmosphere 16, such as argon. As the feed stock is moved forward by the rotating motion of a screw 18, it is heated by heaters 20 and stirred and sheared by the action of the screw 18. This heating and shearing is done to bring the feedstock material into its solid plus liquid temperature range. The thixotropic slurry formed by this action passes through a nonreturn valve 22 in the forward part of the injection system 14 of the machine 10 into an accumulation chamber 24. Upon accumulation of the needed amount of slurry in the accumulation chamber 24, the injection cycle is initiated by advancing the screw 18 with a hydraulic actuator and causing the mold 26 to fill through a nozzle 28. As opposed to other methods of semi-solid molding, the above described method has the advantage of combining slurry generation and mold filling into a single step. It also minimizes the safety hazards involved in melting and casting reactive semi-solid metals. Obviously, and as will be further appreciated, the component construction of the present invention will find applicability as articles, not only in the construction of machines 10 practicing the above method, but also in machines practicing alternative variations on the above process and other processes. Such machines and articles include, without limitation, die casting, metal injection molding, plastic injection molding machines as well as tools and dies.
Because of contact with corrosive semi-solid metals (such as magnesium and zinc), the elevated operating temperatures, oxidation, and the high wear nature of the environment (contact between the various operating parts of the machine and the semi-solid metal is an extremely high wear and shock condition), components of the above machinery are very demanding on their materials of construction. Screw velocities, for example, involve acceleration from 0 to 3 meters/sec. and deceleration back down to 0, all in 0.2 seconds. The chosen materials of construction must be resistant to corrosive attack by the semi-solid metal being processed, must be highly resistant to wear, and must exhibit sufficient strength and toughness to withstand the stresses imposed during long-term exposure at the relevant elevated temperatures under these severe thermal cycling and high shock conditions.
From a corrosion standpoint, iron and some cobalt-based alloys have been reported as satisfactory for processing semi-solid magnesium-base alloys. Nickel-base alloys, such as Alloy 718, are of interest as construction materials because of their good strength at elevated temperatures and lower cost when compared to most cobalt-base alloys. However, because molten magnesium attacks nickel containing alloys, some SSM processors have specified that alloys which come into contact with molten magnesium must contain less than about three percent nickel. Prior machines avoided this problem by using Alloy 718 in their barrel constructions while incorporating a shrink-fitted barrel insert made of a cobalt-base alloy such as Stellite 6 (nominally 28 Cr, 4.5W and 1.2C) or Stellite 12 (nominally 30Cr, 8.3W and 1.4C), which are commercially available from the Cabot Corporation, Kokomo, Ind. While generally performing well with respect to corrosion, they are deficient in toughness and have exhibited cracking and fracture in the machines of the above type. Under the high temperature fatigue conditions of the machines, cracks in Stellite liners have been seen to propagate into the Alloy 718 barrel resulting in total failure of the barrel assembly. This is unsafe and necessitates costly repairs and replacements. It has come to be determined that articles of an alternative material, having greater toughness, would be more desirable in that they would provide for longer wearing components.
The selection of materials for processing semi-solid aluminum-base alloys is much more complex. This is particularly true because most iron, cobalt, and nickel-base alloys are readily attacked by aluminum alloys. In addition to these concerns and those recited in connection with processing magnesium, other important concerns relate to the availability, cost and manufacturing characteristics of the construction material.
In the injection molding of magnesium-base alloys, the maximum operating temperatures within the barrel typically range between about 1100 and 1200.degree. F. with the temperatures sometimes ranging to 1500.degree. F. Most common AISI iron-base hot work tool steels (such as H-10 and H-13, and even more highly alloyed hot work tool steels such as H-19 and H-21) lose strength, hardness and wear resistance at these temperatures. As a result, a number of very specialized materials for machine construction have been used, in particular these alloys include Stellite 6 and 12 (mentioned above) and similar Co--Cr--W--C-type alloys. These alloys have been used to form centrifugally-cast barrel liners or weld overlays. The use of Co--Cr--W--C-type barrel liners avoids the corrosion problems that can be encountered between molten magnesium and nickel-base alloys. Their use as liners therefore permits the use of the more cost effective nickel-base alloys, such as Alloy 718, for barrel construction. Special maraging-type hot work tool steels, such as Thyssen 1.2888 (nominally 0.2C, 10Cr, 2Mo, 5.5W, and 10.00Co) have been used in screws and nonreturn valves. Thyssen 1.2888 reportedly can be used for short times at temperatures as high as 1292.degree. F. (700.degree. C.).
Because of problems related to cost and availability, as well as in an attempt to upgrade performance, the present inventors began a search for a new alloy to replace the currently used Co--Cr--W--C-type alloys and Thyssen 1.2888. This search has lead to Alloy 718 barrels HIP-clad with a new, powder metallurgy (PM) cobalt-base wear resistant alloy, as well as to the construction of various monolithic parts made of the same alloy. The properties of the present components made from PM cobalt-base wear resistant alloys, produced by nitrogen atomization and hot isostatic pressing (HIP), differ considerably from the previously seen Co--Cr--W--C-type alloys (produced from powder by conventional press and sintering methods). The new alloys exhibit an improved combination of strength, toughness, and dimensional stability and it has been found beneficial to also modify their heat treatment.
The traditional Co--Cr--W--C-type alloys are quaternary cobalt-base alloys containing about 27-29% chromium, a variable amount of tungsten (4 to 17%) and carbon (0.9-3.2%). They are widely used in wear resistant applications because of their high strength, corrosion resistance, and ability to retain their hardness at elevated temperatures. Because of their limited hot workability and machinability, however, most of the higher carbon Co--Cr--W--C-type alloys are used in the form of castings, hard facing consumables and powder metallurgy parts.
Considerable work has been done to explore the production of atomized powder metallurgy (PM) Co--Cr--W--C-type alloys by hot isostatic pressing (HIPing) of gas atomized prealloyed powders. In general, prior studies have shown that PM processing of these materials produces a material with higher hardness, higher tensile strength, and higher ductility than is achieved by casting the alloys and that these improvements are still retained at elevated temperature. The abrasive wear resistance of these PM materials is somewhat lower than their cast counterparts owing to the smaller sizes of the primary carbides. For the same reasons, their machinability has been seen to improve.
With regard to the properties of the prior Co--Cr--W--C-type alloys, it has often been assumed that these alloys are at their maximum hardness in the cast or welded condition and that their properties cannot be changed by subsequent heat treatment. Similarly, it has also been assumed that putting these alloys in service at elevated temperature has little effect on their hardness, toughness, and dimensional stability. Contrary to this assumption, some of the published literature for weld deposits, wrought alloys and PM alloys indicate that many of the Co--Cr--W--C-type alloys exhibit an increase in hardness due to carbide precipitation when heated in the range of 1200 to 1500.degree. F. When aged at these temperatures, articles of the Co--Cr--W--C-type alloys might therefore be subject to a change in size, strength and toughness and this is not acceptable in all applications. The operating temperatures during single step, metal injection molding (as generally described above) approach those at which carbide precipitation may occur in PM Co--Cr--W--C-type alloys. Much of the work done and resulting in the present invention was based on concerns about the possible effects that high temperature exposure might have on the mechanical properties and dimensional stability of PM Co--Cr--W--C-type alloys. This research was also conducted to determine what, if any, changes in the alloy composition or subsequent heat treatment could be employed to minimize the above effects on the resulting articles.
In view of the above and other limitations of the prior art, it is a principle object of the present invention to provide fully dense articles made from a novel PM Co--Cr--W--C-type alloy which are highly resistant to changes in size, hardness, corrosion resistance, strength and toughness as a result of prolonged exposure to temperatures in the range of about 1200.degree. F. to 1500.degree. F.
Another object is to provide a fully dense PM cobalt-base article which is resistant to corrosion in semi-solid magnesium and zinc.
It is also an object of the present invention to provide fully dense PM cobalt-base articles which exhibits adequate hardness without a decrease in toughness.
A still further object of this invention is to provide fully dense PM cobalt base articles exhibiting increased toughness over prior art articles and alloys thereby resulting in components of longer life and increased safety.